1. Field of the Invention
The present invention relates to a high-frequency circuit element for the microwave band and, more particularly, to a high-frequency element for use in a circulator, isolator, or inductor, for example.
2. Description of the Prior Art
There has been an increasing demand for size reduction and thickness reduction of high-frequency circuit elements as circulators, isolators, and inductors. In compliance with this demand, the inventors of the present application disclosed a new method for producing a high-frequency circuit element in Laid-open Japanese Patent Application No. 6-61708. This method consists of printing a transmission wire on the surface of a microwave magnetic substrate, laminating a plurality of the substrates by pressing, and firing the laminate. This method permits the production of small, thin high-frequency circuit elements and also permits the accurate alignment of substrates. A high-frequency circuit element produced by this method has the structure as shown in FIG. 3.
The high-frequency circuit element 20 shown in FIG. 3 is composed of a microwave magnetic substrate 1a, three pieces of microwave magnetic substrate 1b each having a transmission wire 2 formed on its principal surface, and two pieces of hard magnetic substrate 3, all the substrates being laminated one over another. The high-frequency circuit element 20 is used as an isolator or circulator in the microwave band.
The high-frequency circuit element 20 is produced by the process explained below with reference to FIG. 4. First, an yttrium-iron powder is prepared as a starting material for the microwave magnetic body. This powder is mixed with an organic solvent, binder, dispersing agent, and plasticizer to give a slurry. This slurry is formed into a strip of green sheet (10-200 .mu.m thick) by the doctor blade method. Green sheets for substrate la and the substrate 1b are formed separately. The green sheet for substrate 1a is cut into several pieces, each serving as green sheet 1a' for the microwave magnetic body. (Green sheet 1a' becomes substrate la upon firing.) The green sheet for substrates 1b has its principal surface printed with a conductive paste to form a transmission line 2 thereon. The green sheet is cut into several pieces, each serving as green sheet 1b' for the microwave magnetic body. (Green sheet 1b' becomes substrate 1b upon firing.) Desired pieces of green sheet 1a' and green sheet 1b' are laminated one over another under pressure, and the laminate is fired to obtain a sintered body (not shown) consisting of several pieces of substrate 1a and substrate 1b.
Secondly, a strontium-iron powder is prepared as a starting material for the hard magnetic body plate 3. This powder is mixed with an organic solvent, binder, dispersing agent, and plasticizer to give a molding material. This molding material is formed into a strip of green sheet 3' (10-200 .mu.m thick) by extrusion. The green sheet is cut to a prescribed size and then fired to give the hard magnetic body plate 3.
Thirdly, the laminate sintered body consisting of several pieces of substrate la and substrate 1b is sandwiched between two pieces of hard magnetic body plate 3. The entire assembly is enclosed in a casing (not shown). Finally, the hard magnetic body plate 3 is magnetized. In this way the high-frequency circuit element 20 is obtained.
The disadvantage of the conventional method for producing the high-frequency circuit element 20 is difficulties in firing simultaneously the green sheets 1a' and 1b' of microwave magnetic body and the green sheet of high magnetic body plate 3. This is due to the fact that the firing temperature of the yttrium-iron-containing substrates 1a and 1b is about 1500.degree. C., whereas that of the strontium-iron-containing hard magnetic body plate 3 is 1250.degree. C. This makes it necessary to perform firing separately on the laminate of green sheets 1a' and 1b' and the green sheets of the hard magnetic body plate 3 and to join the sintered bodies together afterward by an appropriate method. The joining is liable to cause a misalignment of the sintered bodies in the individual high-frequency circuit element. Another disadvantage is that when the laminate of green sheets 1a' , 1b' , and 3' formed by pressing is fired at, say, 1500.degree. C., firing causes the strontium ions contained in the hard magnetic body plate 3 to diffuse into the substrates 1a and 1b, thereby greatly deteriorating the electric properties of the high-frequency circuit element.